Bipyridinium compound/nitrosubstituted n-heterocyclic compound desensitized silver halide emulsion

ABSTRACT

A negative silver halide emulsion is desensitized with a desensitizing combination of a bipyridinium salt and an organic nitrosubstituted N-heterocyclic desensitizer. The emulsion is effectively desensitized for handling in roomlight and the sensitometric properties of the desensitized emulsion are improved with respect to emulsions desensitized with, for example, known organic nitrosubstituted N-heterocyclic desensitizers.

United States Patent John A. Haeiner Rochester, N.Y.

Mar. 2, 1970 Dec. 21, 1971 Eastman Kodak Company Rochester, N.Y.

inventor Appl. No. Filed Patented Assignee BIPYRIDINIUMCOMPOUND/NITROSUBSTITUTED N- HETEROCYCLIC COMPOUND DESENSITIZED SILVERHALIDE EMULSION 9 Claims, No Drawings U.S.Cl 96/10] Int. Cl G03c 1/36Field of Search 96/l0l References Cited UNITED STATES PATENTS 1l/l962Hillson 7 96/10] 3,124,458 3/1964 Fry etal 96/ lOl FOREIGN PATENTS946,476 1/1964 GreatBritain 96/l0l Primary ExaminerJ. Travis BrownAssistant Examiner-Won H. Louie, Jr. Attorneys-W. H. .l. Kline, B. D.Wiese and Henry E. Byers BIPYRIDINIUM COMPOUND/NITROSUBSTITUTED N-HETEROCYCLIC COMPOUND DESENSITIZED SILVER HALIDE EMULSION The presentinvention relates to novel photographic emulsions and elements. Moreparticularly, the invention relates to a new class of desensitizednegative silver halide emulsions, to photographic elements provided withsuch emulsions, and to novel desensitizing combinations for suchmaterials.

It is known that negative silver halide emulsions can be desensitized byincorporating certain agents, called desensitizers, in the emulsion.These agents achieve a reduction in sensitivity by a factor of more than1.0 log E. However, in addition to reducing the sensitivity of theemulsion, these agents cause a change in the sensitometric curve for theemulsion. The desensitized emulsions exhibit, for example, less contrastthan the nondesensitized emulsion. The desensitized emulsions are widelyuseful, particularly in the field of graphic arts and engineeringdrawing reproduction for halftone and linecopying applications whereadvantage can be taken of the roomlight-handling properties of adesensitized emulsion or photographic element. However, it would be adistinct advantage if the contrast of the desensitized emulsion could beincreased. It would thus be desirable to obtain a desensitized emulsionhaving a higher contrast than other emulsions desensitized to the sameextent.

It is, accordingly, an object of the present invention to provide adesensitized silver halide photographic emulsion which has improvedsensitometric properties relative to conventional desensitizedemulsions. It is a further object of the present invention to providephotographic elements including such photographic emulsion. Otherobjects of the invention will be apparent from this disclosure and theappended claims.

I have discovered that a negative silver halide photographic emulsioncan be effectively desensitized with a novel desensitizing combinationcomprising: a bipyridinium salt having two pyridinium nuclei joined by alinkage which is attached to a carbon atom of each of the pyridiniumnuclei; and an organic nitrosubstituted N-heteroeyclic desensitizer. Byproviding this novel combination of materials in a negative silverhalide photographic emulsion, l have achieved a reduction in sensitivityof more than 1.0 log E. In addition, the emulsion exhibits highercontrast, lower minimum density, and a sensitometric curve having a lesslinear toe than conventionally desensitized emulsions.

Examples of the bipyridinium salts useful herein have two pyridiniumnuclei joined by a linkage which is attached to carbon atom of each ofsaid pyridinium nuclei. For example, the two pyridinium nuclei can bejoined by any suitable linkage, such as a single covalent bond, avinylene linkage or an ethylene linkage. Typical bipyridinium saltswhich can be used in the practice of this invention have one of thefollowing formulas:

wherein n represents a positive integer of from 1 to 2; m represents apositive integer of from 2 to 4; R represents a vinylene group (CH= CH-)or an ethylene group CH,CH,-); R and R, each represents an alkyl group(preferably a lower alkyl containing from one to four carbon atoms),e.g., methyl, ethyl, propyl, isopropyl, butyl, hexyl, cyclohexyl, decyl,dodecyl, etc., and substituted alkyl groups,

(preferably a substituted lower alkyl containing from one to four carbonatoms), such as a hydroxyalkyl group, e.g., hydroxyethyl,w-hydroxybutyl, etc., an alkoxyalkyl group, e.g., fi-methoxyethyl,w-butoxybutyl, etc., a carboxyalkyl group e.g., B-carboxyethyl,w-carboxybutyl, etc., a sulfoalkyl group, e.g., B-sulfoethyl,m-sulfobutyl, etc., a sulfatoalkyl group, e.g., B-sulfatoethyl,w-sulfatobutyl, etc., an aroyloxy alkyl e.g., phenacyl, an acyloxyalkylgroup, e.g., fi-acetoxyethyl, yacetoxypropyl, w-butyryloxybutyl, etc.,an alkoxy carbonylalkyl group, e.g., fl-methoxycarbonylethyl,m-ethoxycarbonylbutyl, etc., or an aralkyl group, e.g., benzyl,phenethyl, etc., and the like; or, any aryl group, e.g., phenyl, tolyl,naphthyl, methoxyphenyl, chlorophenyl, etc.; R, and R each represents ahydrogen atom, a lower alkyl group containing from one to four carbonatoms, e.g., methyl, butyl, etc., a lower alkoxy group containing fromone to four carbon atoms, e.g., methoxy, butoxy, etc., or a halogenatom, e.g., chlorine or bromine; R and R each represents a hydrogen atomor a lower alkyl below.

TABLE 1 A. 1,1 '-Dibutyl-4,4'-bipyridinium dibromide l ,l '-Dibutyl-4,4-bipyridinium diperchlorate l,l '-Di(Z-hydroxyethyl)-4,4'-bipyridiniumdibromide l,l '-Diphenacyl-4,4'-bipyridinium dibromide B. C. D. E.l,2-Di( 4-pyridyl )ethylene bis( phenacyl bromide) i i ,imi-pyridyfiethylene bis(phenethobromide) G. Dianhydrol ,2-di[ l(3-sulfopropyl)-4-pyridyl lethylene dihydroxide l. l,l'-Dimethyl-4,4'-bipyridinium dibromide J. l,2-Di(4-pyridyl)ethanebis(phenethobromide) mmomcm-N om-om )gomom 0,11, 2Br

K. l ,l '-Ethylene-2,2'-bipyridinium dibromide al., U.S. Pat. No.3,035,9l7, patented May 22, 1962; Fry et al., U.S. Pat. No. 3,124,458,patented Mar. l0, l964; and E. N. Mason and Sons Ltd. et al., BritishPat. No. 714,290, dated Aug. 25, 1954.

Examples of the organic nitrosubstituted N-heterocyclic: desensitizcrsuseful in the invention include nitrosubstituted thiazoles and compoundsincluding a benzene nucleus such as benzothiazole, quinoline,indolenine, benzotriazole and rhodanine compounds having one or morenitro groups at tached to the benzene nucleus which is either a part ofthe heterocyclic moiety or attached to it through a doubly bonded carbonto carbon chain. The quaternary salts of the nitrosubstituted thiazoles,benzothiazoles, quinolines and indolenines are also useful. Thecompounds include an N-heterocyclic moiety and are substituted with oneor more nitro groups either in the heterocyclic ring or in the benzenering of these compounds which include a benzene ring. The compounds maybe otherwise unsubstituted or substituted with noninter feringsubstituents such as alkyl, preferably lower alkyl of one-three carbonatoms, halo, alkoxy, preferably lower alkox-- y of one-three carbonatoms, mercapto, amino, alkyl amino, preferably lower alkyl amino ofone-three carbon atoms, and the like. The heterocyclic moiety mayinclude a single hetero cyclic: nitrogen atom such as thiazole,benzothiazole, quinoline, indolenine and rhodanine, or a plurality ofheterocyclic nitrogen atoms such as benzotriazole, and may include ahetero sulfur atom such as thiazole, benzothiazole and rhodanine.Representative compounds include 2-mercapto-4 methyl-S-nitrothiazole,2-rnethyl-6-nitrobenzothiazole methop-toluene sulfonate,2-(o-nitrostyryl)--3 -ethylbenzothiazolium iodide and the mandp-nitrostyryl compounds, 2-(o,pdinitrostyryl)-3-ethylbenzothiazoliumiodide, 2(o,p-dinitrostyryl)-benzothiazole, 2-(p-nitrostyryll-quinolinemetho-ptoluene sulfonate, the 6-ethoxy derivative thereof and the oandmnitrostyryl compounds, 4-(p-nitrostyryl)-quinoline methiodide,3,3-dimethyl-2-(p-nitrostyryl)-indolenine methiodide,Z-(p-dimethylarninostyryl)-3-ethyl-6- nitrobenzothiazoliurn chloride,6-nitrobenzo-l,2,3-triazole and the 4- or S-chloro derivatives thereof,and S-o-nitrobenzylidene-3-ethylrliodanine.

For example, the bipyridinium compound and nitrosubstituted compound areincorporated into a negative silver halidle emulsion which may be of anyconventional type,

The novel desensitizer combination according to the present invention isincorporated into the emulsion at any time prior to coating the emulsionon a support. Conveniently the materials are blended into the finishedemulsion prior to coating.

The silver halide emulsions used with this invention can comprise silverchloride, silver bromide, silver bromoiodide, silver chlorobrornoiodideor mixtures thereof. The emulsions may be coarse or fine grain and canbe prepared by any of the well'known procedures, e.g., single jetemulsions, double jet emulsions, such as Lipprnann emulsions, ammoniacalemul sions, thiocyanate or thioether ripened emulsions such as thosedescribed in Nietz et al., U.S. Pat. No. 2,222,264; lllingsworth US.Pat. No. 3,320,069; and McBride U.S. Pat. No. 3,27l,l57. Surface imageemulsions may be used or internal image emulsions such as thosedescribed in Davey et al., U.S. Pat. No. 2,592,250; Porter et al., U.S.Pat. No. 3,206,3l3; Berriman U.S. Pat. No. 3,367,778 and Bacon et al.,US Pat. No. 3,447,927. If desired, mixtures of surface and internalimage emulsions may be used as described in Luckey et al., US. Pat. No.2,996,382. Negative type emulsions may be used or direct positiveemulsions such as those described in Leermakers U.S. Pat. No. 2,l84,0l3;Kendall et al., U.S. Pat. No. 2,54l,472; Berriman U.S. Pat. No.3,367,778; Schouwenaars British Pat. No. 723,0l9; lllingsworth et al.,French Pat. No. 1,520,821; lves U.S. Pat. No. 2,563,785; Knott et al.,U.S. Pat. No. 2,456,953 and Land US. Pat. No. 2,86l,885. The emulsionsmay be regular grain emulsions such as the type described in Klein andMoisar, J. Phat. Sui, Vol. 12, No. 5, Sept/Oct, 1964, pp. 242-251.

The emulsions used with this invention may be sensitized with chemicalsensitizers, such as with reducing agents; sulfur, selenium or telluriumcompounds; gold, platinum or palladium compounds; or combinations ofthese. Suitable procedures are described in Sheppard et al., U.S. Pat.No. 1,623,499; Waller et al,, U.S. Pat. No. 2,399,083; McVeigh U.S. Pat.No. 3,297,447; and Dunn U.Sv Pat. No. 3,297,446.

The silver halide emulsions used with this invention may contain speedincreasing compounds such as polyalkylene glycols, cationic surfaceactive agents and thioethers or combinations of these as described inPiper U.S. Pat. No. 2,886,437; Dann et al., U.S. Pat. No. 3,046,l34;Carroll et al., US. Pat. No. 2,944,900; and Goffe US. Pat. No.3,294,540.

The silver halide emulsions used in the practice of this invention canbe protected against the production of fog and can be stabilized againstloss of sensitivity during keeping. Suitable antifoggants andstabilizers each used alone or in combination include thiazolium saltsdescribed in Brooker et al., U.S. Pat, No. 2,l3l,038 and Allen et al.,U.S. Pat. No. 2,694,7l6; the azaindenes described in Piper U.S. Pat. No.2,886,437 and Heimbach et al., U.S. Pat. No. 2,444,605; the mercurysalts as described in Allen et al., US. Pat. No. 2,728,663; the urazolesdescribed in Anderson et al., US, Pat. No. 3,287,135; the sulfocatecholsdescribed in Kennard et al., US. Pat. No. 3,236,652; the oximesdescribed in Carroll et al., British Pat. No. 623,448; nitron;nitroindazoles; the mercaptotetrazoles described in Kendall et al., U.S.Pat. No. 2,403,927; Kennard et al., U.S. Pat. No. 3,266,897 and Luckeyet al., U.S. Pat. No. 3,397,987; the polyvalent metal salts described inJones U.S. Pat. No. 2,839,405; the thiuronium salts described in Herz etal., U.S. Pat. No. 3,220,839; the palladium, platinum and gold saltsdescribed in Trivelli et al., U.S. Pat. No. 2,566,263 and Yutzy et al.,U.S. Pat. No. 2,597,915.

The photographic elements of this invention may contain incorporateddeveloping agents such as hydroquinones, catechols, aminophenols,3-pyrazolidones, ascorbic acid and its derivatives, reductones andphenylenediamines. Combinations of developing agents can be employed inthe practice of the invention. The developing agents can be in a silverhalide emulsion and/or in another suitable location in the photographicelement. The developing agents may be added from suitable solvents or inthe form of dispersions as described in Yackel U.S. Pat. No. 2,592,368and Dunn et al., French Pat. No. l,505,778.

The photographic and other hardenable layers used in the practice ofthis invention can be hardened by various organic or inorganichardeners, alone or in combination, such as the aldehydes, and blockedaldehydes, ketones, carboxylic and carbonic acid derivatives, sulfonateesters sulfonyl halides and vinyl sulfonyl ethers, active halogencompounds, epoxy compounds, aziridines, active olef'ms, isocyanates,carbodiiniides, mixed function hardeners and polymeric hardeners such asoxidized polysaccharides like dialdehyde starch and oxyguargum and thelike.

The photographic emulsions and elements described in the practice ofthis invention can contain various colloids alone or in combination asvehicles, binding agents and various layers. Suitable hydrophilicmaterials include both naturally occurring substances such as proteins,for example, gelatin, gelatin derivatives, cellulose derivatives,polysaccharides such as dextran, gum arabic and the like; and syntheticpolymeric substances such as water soluble polyvinyl compounds likepoly(vinylpyrrolidone), acrylamide polymers and the like.

The described photographic emulsion layers and other layers of aphotographic element employed in the practice of this invention can alsocontain alone or in combination with hydrophilie, water permeablecolloids, other synthetic polymeric compounds such as dispersed vinylcompounds such as in latex form and particularly those which increasethe dimensional stability of the photographic materials. Suitablesynthetic polymers include those described, for example, in Nottorf US.Pat. No. 3,142,568, issued July 28, 1964; White U.S. Pat. No. 3,193,386,issued July 6, 1965; l-louck et al., U.S. Pat. No. 3,062,674, issuedNov. 6, 1962; Houck et al., U.S. Pat. No. 3,220,884, issued Nov. 30,1965; Ream et al., U.S. Pat. No. 3,287,289, issued Nov. 22, 1966; andDykstra thiazoles, selenazoles and imidazoles. Such nuclei may containalkyl, alkylene, hydroxyalkyl, sulfoalkyl, carboxyalkyl, aminoalkyl andenamine groups and may be fused to carbocyclic or heterocyclic ringsystems either unsubstituted or sub- U.S. Pat. No. 3,41 1,91 l, issuedNov. 19, 1968; particularly ef- 5 stituted with halogen, phenyl, alkyl,haloalkyl, cyano, or alfective are those water-insoluble polymers ofalkyl acrylates koxy groups. The dyes may be symmetrical orunsymmetrical and methacrylates, acrylic acid, sulfoalkyl acrylates orand may contain alkyl, phenyl, enamine or heterocyclic submethacrylates,those which have cross-linking sites which 'stituents on the me hine orpolymethine chain. facilitate hardening or curing, those havingrecurring sul- The merocyanine dyes may contain the basic nucleimenfobetaine units as described in Dykstra Canadian Pat. No. tionedabove as well as acid nuclei such as thiohydantoins, 774,054.rhodanines, oxazolidenediones, thiazolidenediones, barbituric Thephotographic layers and other layers of a photographic acids,thiazolineones, and malononitrile. These acid nuclei element employedand described herein can be coated on a may be substituted with alkyl,alkylene, phenyl, carboxyalkyl, wide variety of supports. Typicalsupports include cellulose sulfoalkyl, hydroxyalkyl, alkoxyalkyl,alkylamlno groups, or nitrate film, cellulose ester film, poly(vinylacetal) film, heterocyclic nuclei. Combinations of these dyes may beused, polystyrene film, poly(ethylene terephthalate) film, polycarifdesired. in addition, supersensitizing addenda which do not bonate filmand related films or resinous materials, as well as absorb visible lightmay be included, for instance, ascorbic glass, paper, metal and thelike. Typically, a flexible support is acid derivatives, azaindenes,cadmium salts, and organic sulemployed, especially a paper support,which can be partially 2o fonic acids as described in McFall et al.,U.S. Pat. No. acetylated or coated with baryta and/or an alpha-olefin2,933,390 and Jones et al., U.S. Pat. No. 2,937,089. polymer,particularly a polymer of an alpha-olefin containing The various layers,including the photographic layers, emtwo to 10 carbon atoms such aspolyethylene, polypropylene, ployed in the practice of this inventioncan contain light abethylenebutene copolymers and the like. sorbingmaterials and filter dyes such as those described in The photographiclayers employed in the practice of this in- Sawdey U.S. Pat. No.3,253,921; Gaspar U.S. Pat. No. vention may contain surfactants such assaponin; anionic com- 2,274,782; Silberstein et al., U.S. Pat. No.2,527,583 and Vanpouncls such as the alkyl aryl sulfonates described inBald Campen U.S. Pat. No. 2,956,879. if desired, the dyes can be siefenU.S. Pat. No. 2,600,831; amphoteric compounds such mordanted, forexample, as described in Jones et al., U.S. Pat. as those described inBen-Ezra U.S. Pat. No. 3,133,816; and No. 3,282,699. water-solubleadducts of glycidol and an alkyl phenol such as Photographic elementsprepared according to this invention those described in Olin MathiesonBritish Pat. No. 1,022,878. can be processed by various methodsincluding processing in The photographic elements employed in thepractice f th alkaline solutions containing conventional developingagents invention may contain matting agents such as starch, titanium h ahydroquinones, catechols, aminophenols, 3- diOXide, Zinc OXide. Silica,p lyme c beads including beads f' pyrazolidones, phenylenediamines,ascorbic acid derivatives, the type described in Jelley ct al.,U.S. Pat.No. 2,992,101 and hydroxylamines, hydrazines, reductones and the like;web y 2,701,245- processing such as described in Tregillus et al., U.S.Pat. No. The Photographic elements used in this invention y 3,l79,517;stabilization processing as described in Russell ct brightening agentsincluding slllbenes, ttiazines. OXaZOleS al., "Stabilization Processingof Films and Papers," PSA Jourand coumarin brightening s Water-Solublebrightening nal, Vol. 16B, Aug. 1950; monobath processing as describedagents may be used such as those described in Albers et al., 40 in LevyCombined Development and Fixation f p German Pat. No. 972,067 and McFallet al., U.S. Pat. No. graphic [mazes i Monobaghs, p Sci. and Eng V L 22,993,390 or dispersions of brishteners may be used such as No. 3, Oct.1953, and Barnes et al., u.s. Pat. No. 3,392,019. 1f P described inJansen German 1,150,274 desired, the photographic elements of thisinvention can be Oetrker et al., U.S. Pat. No. 3,406,070 and HeldkeFrench Pat. processed in hardening developers such as those described inAllen et al., U.S. Pat. No. 3,232,761; in roller transport Swen-a] f fdyes P be F confer processors such as those described in Russell et al.,U.S. Pat. addmonal sensitivity to the light sensitive silver halideemul- 3,025,779; or by surface application processing as sion of themultilayer photographic elements of the invention. described in example3 f Kit e US. Pat. No. 3,418,132. For instance, additional spectralsensitization can be obtained The foliowing examples are included for afurther by treating the emulsion with a solution of a sensitizing dye indemanding ofthe invention an organic solvent or the dye may be added inthe form of a dispersion as described in Ownes et al., British Pat. No.1,154,781. For optimum results, the dye may either be added EXAMPLES F3to the emulsion as a final step or at some earlier stage.

Sensitizing dyes useful in sensitizing such emulsions are To a Portionof a finished convemlmla' mgauve gelatinv described, for example, inBrooker et al., U.S. Pat. No. silver chloride emulsion containing aconventional hardener 2,526,632, issued Oct. 24, 1950; Sprague U.S. Pat.No. and coating aid is added a desensitizing combination accord-2,503,776, issued Apr. 1 l, 1950; Brooker et al., U.S. Pat. No. ing tothe invention in the amount shown below. The emulsion 2,493,748; andTaber et al., U.S. Pat. No. 3,384,486. Spectral is then coated on acellulose acetate film support at 410 mg. sensitizers which can be usedinclude the cyanines, merosilver per square foot and 418 mg. gelatin persquare foot. A cyanines, complex (tri or tetranuclear) merocyanines,comseparate portion of the emulsion is coated in the same manner plex(tri or tetranuclear) cyanines, holopolar cyanines, styryls, without thedesensitizers as a control. Samples of each coating hemicyanines (e.g.enamine hemicyanines), oxonols and are exposed for 10 seconds to a blacklight ultraviolet lamp hemioxonols. through a step wedge, developed in aconventional developer, Dyes of the cyanine classes may contain suchbasic nuclei as and fixed, washed, and dried, all in a conventionalmanner. the thiazolines, oxazolines, pyrrolines, pyridines oxazoles, Thefollowing results are obtained:

Minimum A Rel. Addenda, mgJmole Ag density Gamma log E speed Example:

1 Control 0. 10 1.82 224 2 B (200) plus P 0.04 7.00 -2. 1 2.8 3 B (100)plus R (100)-.- 0.04 7.60 1.6 0.4

B =1, 1'-d1-n-buty1-4, 4-blpyridln1um dihromlde. P 2-mereapto-4-methyl-b-nltrothlazolo. R= 2-rnothyl-d nltrobenzothlazolomotho-p-toluono sulionato.

Examples 2 and 3 show that the emulsion is desensitized more than 1.0log E by the addition ofa novel desensitizer combination according tothe invention. The desensitized emulsion furthermore has excellentsensitometric properties as shown in the table. Other combinations ofthe bipyridinium compounds and nitrosubstituted compounds mentionedabove achieve similar results.

EXAMPLES 4-6 results are as follows:

Addenda Minimum Rel. Example mgJmole Ag density Gamma Speed P I) 5 PI00) 0.03 4.9 200 6 8(200) 0.03 6.2 42

A similar improvement in sensitometric characteristics, with excellentdesensitizing, is achieved with other novel desensitizer combinationsaccording to the invention.

The amount of desensitizer combination added to the emulsion can varywisely from the amounts shown in the examples. ln general, an amount isused such that densitizing to an extent of more than 1.0 log E isachieved, and improved sensitometric properties are achieved where theamount of each of the two tubes of material making up the combination ispresent in an amount of 25-75 percent, preferably 35-65 percent, byweight of the combination. The maximum amount of sensitizer combinationcan vary quite widely without deleterious effect, but amounts in excessof about 500 mg. per mole of silver are not usually justifiedeconomically. Therefore, the sensitizer combination will generally bepresent in an amount of not more than 500 mg. per mole of silver. Theminimum amount of sensitizer combination sufficient to ensuredesensitization of l.O log E will vary somewhat but is generally about50 mg. per mole of silver,

EXAMPLES 7 and 8 The following examples show that the addition of awatersoluble cadmium salt increases the maximum density. A silverchloride emulsion (7) is prepared and coated similar to that describedas example 2.

Another silver chloride emulsion (8) is prepared similar to example 2except addenda B (l,l -di-n-butyl-4l,4'-bipyridini um dibromide) islowered to 100 mg. per silver mole and cadmium chloride is added to theemulsion layer at 10 g. per silver mole.

The: above two coatings are then exposed for l minute to a pulse xenonlight source, developed as described in the following table, fixed,washed and dried.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

lclaim: l. A desensitized negative silver halide photographic emulsioncontaining a desensitizing combination comprising a 5 bipyridinium salthaving two pyridinium nuclei joined by a linkage which is attached to acarbon atoms of each of said pyridinium nuclei and an organicnitro-substituted N-heterocyclic desensitizer.

2. A desensitized negative silver halide photographic emulsion accordingto claim 1 wherein said desensitizer combina tion is present in anamount of at least 50 mg. per mole of silver, and said organicdesensitizer is present in an amount of from 25-75 percent by weight ofthe combination and is selected from the group consisting ofnitro-substituted thiazoles and compounds of benzothiazolc, quinoline,indolenine, benzotriazole, and rhodanine having one or more nitro groupsattached to a benzene nucleus of said com pounds, said benzene nucleusbeing either a part of the heterocyclic moiety of said compound orattached thereto through a doubly bonded carbon chain.

3. A desensitized negative silver halide photographic emulsion asclaimed in claim 1 wherein said organic desensitizer is present in anamount of from 35-65 percent by weight of said combination and isselected from the groups consisting of 2-mercapto-4-methyl-5-nitrothiazole, 2methyl-6- nitrobenzothiazolemetho-p-toluene sulfonate, 2-nitorstyryl-3 ethylbenzothiazolium iodide,2-(o,p-dinitrostyryl)-3-ethylbenzothiazolium iodide,2-(o,p-dinitrostyryl)-benzothiazole, Z-(p-nitrostyryl)-quinolinemetho-p-toluene sulfonate, 6- ethoxy-Z-nitrostyryl-quinolinemetho-p-toluene sulfonate, 4- (p-nitrostyryl)-indolenine methiodide,Z-(p-dimethyIaminos' tyryl)-3-ethyl-6-nitrobenzothiazolium chloride,o-nitrobenzo- 1,2,3-triazole, 4-chloro-6-nitrobenzo-l ,2,3-triazole,5-chloroo-nitrobenzo-l,2,3-triazole, and S-o-nitrobenzylideneB-ethylrhodanine.

4. A desensitized negative silver halide photographic emulsion accordingto claim l wherein said descnsitizer combination is present in an amountof at least 50 mg. per mole of silver, the pyridinium component ispresent in an amount of 2575 percent by weight of said combination andwherein the linkage of said pyridinium compound is selected from thegroup consisting of a single covalent bond, a vinylene linkage and anethylene linkage.

5. A desensitized negative silver halide photographic emulsion accordingto claim 4 wherein said bipyridinium salt is present in an amount offrom 35 to 65 percent by weight of said desensitizer combination and isselected from compounds having the formulas:

wherein n represents a positive integer of from 1 to 2', m represents apositive integer of from 2 to 4', R represents vinylene or ethylene; R,and R each represents alkyl, hydroxyalkyl, alkoxyalltyl, carboxyalkyl,sulfoalkyl, sulfatoalltyl, aroyloxy alkyl, acyloxyalkyl,alkoxycarbonyalkyl, aralkyl, and aryl; R and R. each representshydrogen, lower alkyl containing from one to four carbon atoms, loweralkoxy containing from one to four carbon atoms, or halogen; R and Reach represents hydrogen or a lower alltyl group containing from one tofour carbon atoms, and X represents an acid anion.

6. A negative silver halide photographic emulsion according to claim 5wherein said bipyridinium compound is a member selected from the groupconsisting of: l,l -dibutyl 4,4'

bipyridinium dibromide, diperchlorate, 1,1-di(2-hydroxyethyl)-4,4'-bipyridinium, dibromidel,l'-diphenacyl-4,4-bipyridinium dibromide, l,2- di(4-pyridyl)ethylenebis(phenacyl bromide), l,2-di(4-' pyridyl)ethylene bis(phenethobromide),dianhydro-l,2-di[ l- (3-sulfopropyl)-4-pyridyl}ethylene dihydroxidel,2-di(4- pyridyl)ethylene bis(methobromide), l,l'-dimethyl-4,4-bipyridinium dibromide, 1,2-di( 4-pyridyl)ethane bis(phenethobromide 1,1-ethylene-2,2 '-bipyridinium dibromide, andl,l'-tetramethylene-2,2'-bipyridinium dibromide.

7. A negative silver halide photographic emulsion according to claim 1containing, per mole of silver, 50 toS QO mg. of a 1,1-dibutyl-4,4'-bipyridinium

2. A desensitized negative silver halide photographic emulsion accordingto claim 1 wherein said desensitizer combination is present in an amountof at least 50 mg. per mole of silver, and said organic desensitizer ispresent in an amount of from 25-75 percent by weight of the combinationand is selected from the group consisting of nitro-substituted thiazolesand compounds of benzothiazole, quinoline, indolenine, benzotriazole,and rhodanine having one or more nitro groups attached to a benzenenucleus of said compounds, said benzene nucleus being either a part ofthe heterocyclic moiety of said compound or attached thereto through adoubly bonded carbon chain.
 3. A desensitized negative silver halidephotographic emulsion as claimed in claim 1 wherein said organicdesensitizer is present in an amount of from 35-65 percent by weight ofsaid combination and is selected from the groups consisting of2-mercapto-4-methyl-5-nitrothiazole, 2-methyl-6-nitrobenzothiazolemetho-p-toluene sulfonate, 2-nitorstyryl-3-ethylbenzothiazolium iodide,2-(o,p-dinitrostyryl)-3-ethylbenzothiazolium iodide,2-(o,p-dinitrostyryl)-benzothiazole, 2-(p-nitrostyryl)-quinolinemetho-p-toluene sulfonate, 6-ethoxy-2-nitrostyryl-quinolinemetho-p-toluene sulfonate, 4-(p-nitrostyryl)-indolenine methiodide,2-(p-dimethylaminostyryl)-3-ethyl-6-nitrobenzothiazolium chloride,6-nitrobenzo-1,2,3-triazole, 4-chloro-6-nitrobenzo-1,2,3-triazole,5-chloro-6-nitrobenzo-1,2,3-triazole, and5-o-nitrobenzylidene-3-ethylrhodanine.
 4. A desensitized negative silverhalide photographic emulsion according to claim 1 wherein saiddesensitizer combination is present in an amount of at least 50 mg. permole of silver, the pyridinium component is present in an amount of25-75 percent by weight of said combination and wherein the linkage ofsaid pyridinium compound is selected from the group consisting of asingle covalent bond, a vinylene linkage and an ethylene linkage.
 5. Adesensitized negative silver halide photographic emulsion according toclaim 4 wherein said bipyridinium salt is present in an amount of from35 to 65 percent by weight of said desensitizer combination and isselected from compounds having the formulas:
 6. A negative silver halidephotographic emulsion according to claim 5 wherein said bipyridiniumcompound is a member selected from the group consisting of:1,1''-dibutyl-4,4''-bipyridinium dibromide,1,1''-dibutyl-4,4''-bipyridinium diperchlorate,1,1''-di(2-hydroxyethyl)-4,4''-bipyridinium dibromide1,1''-diphenacyl-4, 4''-bipyridinium dibromide,1,2-di(4-pyridyl)ethylene bis(phenacyl bromide),1,2-di(4-pyridyl)ethylene bis(phenethobromide),dianhydro-1,2-di(1-(3-sulfopropyl)-4-pyridyl)ethylene dihydroxide1,2-di(4-pyridyl)ethylene bis(methobromide),1,1''-dimethyl-4,4''-bipyridinium dibromide, 1,2-di(4-pyridyl)ethanebis(phenethobromide), 1,1''-ethylene-2,2''-bipyridinium dibromide, and1,1''-tetramethylene-2,2''-bipyridinium dibromide.
 7. A negative silverhalide photographic emulsion according to claim 1 containing, per moleof silver, 50 to 500 mg. of a desensitizer combination consistingessentially of from 25 to 75 percent by weight of1,1''-di-n-butyl-4,4''-bipyridinium dihalide, and from 25 to 75 percentby weight of a member selected from the group consisting of2-mercapto-4-methyl-5-nitrothiazole and 2-methyl-6-nitrobenzothiazolemetho-p-toluene sulfonate.
 8. A photographic element comprising asupport having thereon a desensitized negative silver halide emulsionlayer as claimed in claim
 1. 9. A photographic element comprising asupport having thereon a desensitized negative silver halide emulsionlayer as claimed in claim 7.